Method of manufacture of sheet material and apparatus for the production thereof



Jan. 25, 1944. A, L DALY ETAL 2,339,840 METHOD 0F MANUFACTURE 0F SHEET MATERIAL AND APPARATUS FOR THETRoDUcTIoN THEREOF Filed July 17, 1940 PatentedJan. 25, 1944 l UNITED STATES PATENT voiilcie;

v METHOD oF MANJSRE oFv SHEET Mln l I I TERIAL AND APPARATUS FOR THE PRO- DUCTION THEREOF of Delaware Application July 1v, 1940, serial No. 346,055

In Great BritainvAugust- 19, 1939 Claims.

This invention relates to the production of sheet'material, especially relatively thick sheet material, of cellulose acetate or like thermQ-.

plastic organic nlm-forming material and to apparatus for the production of such material.

.Sheets of such ,material which are suiiciently thick to be self-supporting are at present made by forming a composition containing a lmforming substance, 'a plasticiser and a volatile solvent into a block `,in a heated.press, cutting sheets of the required thickness from the block, seasoning the sheets to remove volatile solvent and attening the sheets to remove warping occurring during the seasoning operation and nally polishing the sheets to remove4 knife lines.

This process involves elaborate and costly plant in the form of lblock presses, flattening presses and polishing presses and is somewhat slow, and lacking in exibility. Thus, fbr instance, to increase the superilcial dimensions of sheet materials which it is required to produce may involve laying down completely new presses. It is an object of the present invention to provide a. process for the production of such sheet materials which is morerapid, less expensive and -more flexible in the sense that a change in the superficial dimensions of the material produced can be made at any time at a small additional outlay.

We have found that the object referred to above can be attained by casting a solution of the film-forming substance in 'a volatilesolvent containing an appropriate quantity of plasticiser on to the smooth iiat surface of an alloy of low melting point, i. e., of melting point below about 100 C.. .Preferably the sheet material is stripped lfrm the metauic surface wnue it still contains a substantial amount of volatile solvent and resi-dual solvent is then removed by exposing the sheet, held under tension in a tentering device general involve the use of mercury and are prefto a drying atmosphere.

Preferably the sheet is built up to the required thickness from relatively thin layers. Thus, for example, a sheet of 0.05 to 0.1", 0.2" or.`0.3" or more can with advantage be built up from successive layers of thickness ranging from less than 0.005 to 0.05", for example between 0.01 and 0.025". Each layer is allowed'to dry tothe extent of losing the bulk of its volatile solvent before applying the 4next layer'. y

Successive layers need not be of the same thickcontain pigments, dyes or other substances adapted to aiiectthe quality of the light transm'itted, e. g., by polarising it or excluding radia tion of a particular range of wave-length, for instanceI radiation of high actinic value. For some purposes it is of value for one or more outer layers of the composite sheet to .be more water resistant than the inner layers. This can be achieved for instance by forming the inner layers of a. normal cellulose acetate having an acetyl value of say 52`54% .calculated as acetic acid and forming the outer layers of a cellulose acetateI of higher acetyl value. Similarly, if the outer layers are required to be less water resistant than. the inner layers they may be made of a cellulose acetate of low acetyl value. The

film-forming lsubstances in successive layers may be of quite diierent constitution. For'examplev the outer layers may be of .cellulose acetate butyrate and the inner layers of cellulose acetate.

The casting surface, as indicated above, is formed of an alloy of low melting point. The melting point may be considerably below that of the cellulose acetate or other film-forming base from which the sheet material is to be formed. Alloys containing lead, tin, bismuth and cadmium have been found suitable.

Examples of such alloys are the following., the proportions being by Weight:

Number Lead Tin Bismuth Cadmium.

13. 13 49. 5 10. 10 ll. 57. l 8. 6 23. 5 44. l5v 8. 85

Such. alloys have melting points between and or 75 C. Alloys of lower melting point in erably avoided in view of the danger of poisoning involved.

of glass, provided with side pieces of suitable material., so that the glass plate forms the bottom ness or composition. Thus for example pleasing effects may be produced by incorporating metal-A lic powders or other eiect materials in the dope from which the intermediate layers are formed. Some of the layers may be colourless and others of the mould. It is of advantage to chill thesurface of the alloy rapidly, and to this end the glass plate may, with advantage, be separated by a thin lm of water from a metallic table. `Another method of.chilling the alloy rapidly is to lower into it while it is molten inthe glassbottomed mould of the kind described, a steel plate scored in such a. way as to cause the' alloy readily to adhere to the plate. and, after the alloy is set, to raise the steel plate and with it the solid plate of alloy to which a iiat smooth surface will have been imparted by the glass. A further method is to lift a lmof the molten alloy out of the body of the alloy by means of a glass slide having a smooth polished surface.

One form'of apparatus for producing casting plates according to the invention is illustrated The alloy is mated and poured into the mould' formed by the glass plate l and side-pieces 3, to

form a layer 1.

.plasticisers of acetate.

When cellulose acetate is employed as the film- 2,389,840 Y glycollate, triacetin, diethyl tartrate, or dibutyl tartrate. Plasticisers which are adapted to increase the re resistance of the material, for example tricresyl phosphate, triphenyl phosphate, tributyl phosphate, trichloroethyl phosphate and trichlorobutyl phosphate, may also be present, preferably, however, in admixture with forming substance, acetone may with advantage be used as the 'volatile solvent. Other suitable The iron plate 8, has its lower surface 9 scored u at I0 to facilitate adhesion of the fusiblealloy to the plate. The plate is reinforced with webs l I ruiming transversely and I2 running longitudinally. Holes I3 are provided in the webs I2 to facilitate lifting the plate out of the mould.

The plate 8 is lowered into the molten alloy l within the mould so as to chill the alloy rapidly and is then lifted out of the mould with the block of alloy adhering to it. The lower surface of this alloy is flat and has a high polish.

Invuse the assembly of plate 8 and block 'I is inverted, and sheets of the hlm-forming substance are cast on that surface of the block of alloy which was in contact with the glass.

By methods such as those described above, a

lent casting surface for the purpose of the involatile solvents are dioxane and methylene ethylene ether. employed, for example mixtures of a latent solvent such as ethylene dichloride or methylene dichloride in ethyl or methyl alcohol or mixtures containing a true solvent such as acetone or dioxane in admixture with a volatile diluent such as benzene or xylene and a low or medium boiling yliquid of lower solvent power than the acetone' or dioxane, for example ethyl acetate or methyl oriethyl alcohol. High boiling solvents such-as ethyl lactate or diacetone alcohol may also be present. employ 'a simple volatile solvent and it is one of the advantages of the process of the invention that sheets of excellent clarity can be obtained using a simple solvent such las acetone.y This facilitates solvent recovery, which is also facili-k tated by the simplicity of the process and apvention. To obtaina similar surface by polishing a metallic plate is a very laborious and costly operation, which few operatives are sumciently skilled to undertake. A glass surface of equal smoothness can readily be obtained, but the use of glass has certain disadvantages compared with lthat of the low melting alloy. Thus, heat transmission through the glass is much slower. There A then be re-melted and re-cast. Moreover, if desired, the Y sheet of cellulose acetate or the like can be cast on to the surface of the molten alloy. This ensures a perfectly at smooth surface. The

invention includes the production of the casting surface of low melting alloy as well as the use of such a surface in the production of sheet materials of the kind referred to. A

The proportion of plasticiser in the sheet material may with advantage be; between 30 and 40 to 50% based on the weight of the cellulose acetate. This proportion of plasticiser facilitates drying of the sheet material in the initial stages, vpossibly by reducing the tendency to the forma-V tion of a skin' on the surface which hampers evaporation. Any suitable plasticiser for the lmforming material can be employed, for example in the caseof cellulose acetate we may employ dimethyl phthalate, diethyl phthalate, dimethoxy ethyl phthalate, diethoxy ethyl phthalate, methyl phthalyl glycollate, methyl phthalyl methyl paratus. The removal of the bulk of the volatile solvent, can be eiected at ordinary temperatures or at elevated temperatures, for example 30-40 C. or even higher. When working at relatively high temperatures especial care must be taken to ensure freedom of the air and of the casting surface from dust, with a view to avoiding bubble formation.

It is-of advantage to strip the sheet material from the casting surface before completely removing the volatile solvent, e. g. while the material still contains some 10 or 1215% of its weight of solvent. Substantial removal of'the remainder of the volatile solvent can then be effected while the sheet material is stretched on a tentering device in a warm atmosphere.

The solution employed may contain between l0 and 40% by weight of the cellulose acetate or other hlm-forming substance. The concentration may for example, lie between 15 and 25%, or between. 25 and 35%.

The invention has been described with par' ticularreference to the use of cellulose acetate as the nlm-forming substance. Other thermoplastic organic film-forming substances., may, however, be used in the process of the invention, for example other derivatives of cellulose such as cellulose propionate, cellulose butyrate, cellulose acetate-butyrate, cellulose acetate-propionate,-

cellulose acetate-stearate, cellulose nitrate, cellulose nitrate-acetate, cellulose nitrate-propionate and cellulose ethers such as ethyl, propyl and benzyl cellulose. Other thermoplastic organic film-forming 'substances that can be employed include polyvinyl esters, for example polyvinyl greater aflinity for cellulose Solventl mixtures may also be Y It is preferable, however, to

process of the invention is simple and inexpensive both in capital outlay and in operation.

e Process and apparatus can rapidly and inexpensively be turned from the production of sheets of one size to sheets of another size. The casting surface is less easily damaged than a glass surface and can be quickly and cheaply reformed when desired. The alloy is a good heat conductor compared with glass. Production of the solution from. which the sheet materials are castis also simple and does not involve the use of complex solvent mixtures or oflaborious'milling dition the products of the invention have certain advantages over products made by the known method referred to above.

Instead of stripping the sheet from the-'plate n 3 having a melting point higher than that of said alloy removing the bulk' of said solvent and se`paiating thesheet so formed from said surface by heating `to a temperature suicient-to melt the in contact with the at'l polished surface of the glass, a solution of a. thermoplastic organic filmforming substance, selected from the group con- K operations. Solvent recovery is simple. In ad- Thus, they z have greater flexibility and clarity. They can.

after removing the bulk of the volatile solvent, Y

the plate may be removed from the sheet by heating the assembly to a temperature s uicient to melt the alloy without damaging the sheet.

Having described our invention, what we desire to secure by Letters Patent is: 1. Process for the production of a film-casting support, which omprises forming a block of' an Aalloy melting be ow about 100 C., by pouring' -alloy with, the low melting alloy, allowing the alloy to set with the base-plate in position 'jand removingthe block of alloy having said baseplate embedded therein without damaging that surface of said block which was in contact with said glass plate. V

.2. Process for the production of sheet mate-'- l rial, which comprises forming a nlm-casting sup.

port by the process claimed in claim .1, and casting on to that surface of the alloy which .was in" contact with the ilat polished surface of the glass, a solution of a thermoplasticforganic filmsisting of cellulose esters'and synthetic resins, in a volatile solvent, said lm-forming substance having a melting point higher than that ofsaid alloy removing the bulk of said lsolvent and sep;- arating the sheet so formed from said surface by heating to ia temperature sufiicient to melt the alloyv without damaging the sheet material and recasting the alloyzto. form the support for a fresh film-casting operation. w

4. Process for the production of s heet material, which comprises forming a, nlm-casting support by the process claimed in claim 1, and casting on to that surface of 'the alloy which was having a melting point higher than that of said.,

alloy, removing the bulk of said solvent and sep# arating the sheet-so formed from saisi4 surface by heating to a temperature sufcient to melt the alloy without damaging the sheet material.

, heating to a temperature suicient to melt the formingA substance; selected from the group con- A sisting of celluloseesters and synthetiqrsins, in ,a fa volatile solvent, said film-forming substance' 5. Process for the production of sheet material, whichcomprises forming a nlm-casting support by the process claimed in claim 1, and casting on to that surface of the alloy which was in contact with the flat polished surface of the glass, a solution of cellulose acetate in a volatile solvent 'comprising acetone, said cellulose acetate having a melting point higher than' that of said alloy, removing the bulk of said solvent and sep arating the Isheet so formed from said surface by alloy without damaging the sheet material and recasting the alloy to form the support' for a fresh filniecasting operation. 

